The streets of the central area are far from clean. Many public toilets are fairly horrible. Those in hotels and restaurants are better. A good guide will know where the best restrooms are.
Even when there is a language barrier, ordinary people are courteous. However the drivers of St Petersburg are always in a hurry. City streets are wide and only at traffic lights is it (fairly) safe to cross.
Tap water often contains various impurities from an antiquated network of pipes. You should stick to bottled water. Food bought from stalls on the street is usually edible and safe, if not always delicious.
The season of White Nights comes at the end of June, when the sun hardly drops below the horizon and daylight lasts around the clock.
Quite a few festivals and events during this period last late into the night. Daylight is not the same as clear skies and it is wise to have an umbrella with you.
Although St Petersburg is more western-looking than the rest of Russia, foreign languages are not spoken widely. Cyrillic script is an extra complication although, in the city centre, some street signs are now written in Latin too.
Like Americans and Britons, quite a few ordinary Russians seem to believe that simply speaking their language louder will help foreigners grasp the meaning.